"To me, what United[Healthcare] did literally on the verge of enrollment puts them on pretty thin ice in terms of member satisfaction based on some of the calls we've gotten in our office," Courtney said.

He wants to know how a major change to UnitedHealthcare's physician network might affect their bonus payments, if at all.

U.S. Sen. Richard Blumenthal said in a prepared statement, "I am deeply disappointed by UnitedHealthcare's unfortunate decision to limit provider options for Medicare beneficiaries even as health care reform seeks to give patients access to a broad array of qualified health care professionals.

"This shortsighted, sweeping decision will have real-life ramifications for people across our state who may no longer have access to critical health services," Blumenthal said. "I have asked United for an explanation for their decision as quickly as possible."

The Connecticut State Medical Society, the state's largest group of doctors, reached out to the congressional delegation to ask for assistance. The society has also contacted the American Medical Association.

Matthew Katz, CEO of the Connecticut society, said the AMA told him that it is investigating similar cuts to doctor networks for Medicare Advantage patients in other states. For example, Katz said, UnitedHealthcare is apparently cutting doctors from the program in other states.

"It's not just Connecticut," he said.

Katz said the AMA told him that UnitedHealthcare is cutting doctor networks in at least Rhode Island and Florida, where doctors received letters terminating their service, similar to those sent to physicians in Connecticut.

Doctors and elected officials have inquiries into the federal Centers for Medicare & Medicaid Services, which pays insurers to administer Medicare Advantage programs, but that agency is operating with very limited staff because of the federal government shutdown, said Dr. Michael F. Saffir, president of the state medical society.

Even with the cuts, UnitedHealthcare says it has a broad and adequate network for its customers.

UnitedHealthcare spokesman Benjamin Goldstein said via e-mail, "We offer our members one of the most extensive networks of health care providers in Connecticut and remain committed to ensuring they have continued access to quality, affordable care. Members with questions are encouraged to call the customer service number on the back of their member ID card for more information."

Robert Zirkelbach, a spokesman for America's Health Insurance Plans — an insurer trade group, has said that the Affordable Care Act cuts payments to insurers who take part in Medicare Advantage and those cuts "are big and are starting to have a real impact on seniors in the program."

Reduced physician networks were anticipated back in May 2010 when the non-partisan health policy group Kaiser Family Foundation was sorting through the effects of cutting Medicare Advantage payments. In a May 2010 analysis, the foundation wrote that "some companies may decide to raise beneficiaries' premiums and/or cost-sharing requirements, reduce their network of providers, reduce extra benefits, or make improvements to obtain quality-based payments."

There's widespread confusion among doctors in Connecticut about whether they will be in or out of UnitedHealthcare's Medicare Advantage network next year, according to the Connecticut medical society. The society said some doctors received as many as four letters from the insurer, which said they had been "terminated without cause," starting Feb. 1, 2014.

The timing of the announcement couldn't be worse because enrollment for Medicare plans starts Tuesday and ends Dec. 7, said Saffir, the medical society president said.

News broke Tuesday that UnitedHealthcare is cutting its doctor network next year after the Fairfield County Medical Association posted an alert about the action on its website. The Fairfield group said it affects 810 primary care physicians and 1,440 specialists. UnitedHealthcare would not say how many doctors have been cut, but it did say that the remaining network includes more than 1,500 primary care physicians and more than 4,000 specialists.

UnitedHealth Group Inc, the largest U.S. health insurer, reported a slight drop in fourth-quarter profit on Thursday as medical costs rose, but revenue increased more than 11 percent, helped by growth in its Medicare, care management, technology and international businesses.